The present invention relates to a flexible impact and tear resistant composite sandwich plate and construction system for vessels, such as, for example, tankers, bulk carriers or ships for which it is desirable to contain the vessel contents during conditions of extreme or accidental load or for which it is desirable to prevent failure (e.g., sinking of bulk carriers, ruptured members) or for which it is desirable to reduce costly repairs from fatigue cracks; and for civil or maritime structures, such as, for example, orthotropic bridge decks, railway bridges or tanks for which it is desirable to minimize strain concentration, reduce fatigue problems, improve thermal insulation and acoustical insulation or vibration characteristics, or where it is desirable to connect similar or dissimilar metals without welding.
Increased social, economic and political pressure has led to the development of technology to reduce or eliminate the risks of pollution and resulting damage to the marine environment, as well as the loss of valuable cargo, that may result from cargo leaking due to rupture of a vessel under extreme or accidental loads such as collisions, grounding, fire and explosion. In particular, vessels carrying hazardous materials are increasingly subject to additional requirements imposed by regulatory agencies, ship and cargo insurers, and ship owners and financiers. The high cost of hazardous spill liability and increasing cargo values has further encouraged the development of leak and rupture resistant vessels.
One approach to containing vessel contents is the provision of double hulls for oil tankers. An inner cargo containing hull of a stiffened single plate construction is supported within an outer protective hull, which is also a stiffened single plate construction. A conventional double hull has longitudinal and transverse frames between the inner and outer hulls. A more advanced, alternative double hull has only longitudinal frames between the inner and outer hulls, allowing for simplified construction suitable for assembly line production by robotic devices. Both conventional and advanced double hull designs have transverse bulkheads between cargo compartments in the inner hull, and may have bulkheads between ballast compartments which are generally located between the inner and outer hulls. Variations in double hull design include constructions with a double bottom only, or with a double bottom and double hull sides. To reduce weight, the deck is generally a single plate construction. Alternatively, convexly curved hull plates between longitudinal frames may provide high energy absorption in the curved plate double hull.
FIG. 1 shows a cross-section of a typical double hull oil tanker designed according to conventional naval architecture. FIG. 2 illustrates the arrangement of cargo tanks and other sections for a typical double hull vessel.
The advantages of double hull construction over conventional single hull designs are also well known. These advantages include improved cargo handling efficiency, better cargo purity, and reduced water pollution by isolating ballast tanks from cargo holds. Furthermore, double hulls constructed to international standards which require a two meter space between inner and outer hulls also offer reduced risk of leakage or rupture due to penetration of the outer hull during collisions or groundings. The claimed innovative features of advanced double hulls are improved strength, ease of manufacture and reduced welding and steel surface areas in ballast tanks, increased accessibility to ballast tanks which results in better inspection and improved maintenance and inner hull retention of oil during high energy grounding. With current technology, double hull vessels involved in low energy, low velocity impacts are less likely to be compromised and less likely to cause pollution than a single hull vessel. The improved tanker designs, such as double-bottom, double sides, double hull, mid-deck, etc. are known to reduce but not eliminate the risk of oil spills in accidents. Although tests indicate that an advanced all steel double hull design will dissipate more energy than a conventional all steel double hull design, both designs are subject to compromise of the inner hull due to crack propagation resulting from fatigue cracks or from cracks that propagate from a ruptured plate during extreme load events.
Patents related to improving the energy absorption capacity of double hull construction due to accidental or extreme load events such as grounding or collision include U.S. Pat. No. 5,218,919 to Krulikowski III et al. and U.S. Pat. No. 5,477,797 to Stuart. Both patents are directed to retrofitting existing single hull tankers with external hulls to make a double hull tanker. Krulikowski III et al. describe the use of energy absorbing telescoping members arranged in a truss-like formation to support a laminated steel auxiliary hull to the outside of an existing oil tanker hull. Construction details of attachments to transverse bulkheads and deflection control devices are also described. The void between hulls is filled with polyurethane foam/balls to distribute impact forces, to support the auxiliary hull under hydrostatic loads and to provide additional buoyancy in the case where the auxiliary hull is ruptured. Stuart describes the construction of an auxiliary hull attached to the outside hull of an existing oil tanker. It is composed of a series of longitudinally framed steel plates that form a honeycomb configuration, when viewed in section, between the hulls. The combination of stress relief joints, which make the outer hull discontinuous, and the honeycomb inner hull structure create a damage resistant hull. The construction also allows the inner hull space to be flooded to any level to provide the appropriate ballast by means of a pressurized inert gas and a vacuum pressure system. These retrofitted external hull structures fail to address the possibility of crack propagation into the inner hull due to rupture of the outer hull, and inadequately address the cost and practicality of fabrication and maintenance of the auxiliary hull structure. In current retrofit designs, access between the hulls for inspection and corrosion maintenance is difficult, if not impossible. The external hull in a retrofit design generally does not participate in carrying all of the operational loads, and adds significant dead weight to the tanker with limited structural functionality.
U.S. Pat. No. 4,083,318 to Verolme and U.S. Pat. No. 4,672,906 to Asai are directed to LNG (liquid natural gas) tankers and to tankers carrying cryogenic or high temperature freight in which the cargo tanks are separate structures from the tanker and do not form part of the load carrying hull girder system of the tanker.
Current all steel double hull construction has serious disadvantages which lower the likelihood that these design types will meet the performance criteria of zero oil outflow after accidental or extreme load events such as collisions, groundings, explosions or fire, and remain competitive relative to construction, maintenance and service life costs. One disadvantage is that current double hull construction is based on traditional naval architecture design concepts in conjunction with international agreements and national standards that stipulate the use of double hull construction with a minimum separation between hulls which is related to statistical data of measured rock penetrations from recorded tanker casualties.
Hulls constructed according to traditional naval architecture standards generally provide a complex system of steel plates and plate steel structural members, such as frames, bulkheads and girders. The carrying capacity of the steel plates and supporting members is increased by reinforcing the plates and structural members with multiple stiffeners of the type well known in the art, such as flat, angle or channel metal stock fastened to plate surfaces. This complex hull structure and plate stiffener system is a source of fatigue failures and a source for tearing (rupture) of the hull plate during accidental or extreme loads. This type of hull is costly to fabricate due to the large number of pieces which must be cut, handled and welded, and because of the significantly increased surface area on which protective coatings must be applied. Also, these typical complex structural systems are very congested, leading to poor access, poor inspection, poor and costly maintenance, and a decreased service life due to corrosion.
Recent large scale grounding tests on double hull sections also indicate that despite the superiority of double hull vessels over single hull vessels, rupture of the interior hull of currently available steel double hull designs may occur as a result of crack propagation from the initial rupture of the outside hull primarily at or near transverse structural members. The crack initiated in the outside hull propagates through the structural members between the inner and outer hulls and is transmitted to the inner hull. The obvious consequence of inner hull rupture will be oil outflow from each ruptured cargo hold. Providing a crack arrest layer or other structure to prevent the propagation of cracks through the steel structure into cargo tanks is not disclosed in current design alternatives. Therefore, preventing or reducing oil outflow in the event of accidental or extreme load events is not adequately addressed by currently available design alternatives.
A large scale composite steel polyurethane foam sandwich plate has been tested for its ability to prevent leak and rupture of a hull. These tests illustrate that polyurethane foam does not adequately adhere to the steel plates and has little shear strength. Low shear strength minimizes the flexural capacity of the composite and lack of adhesion precludes the possibility of using polyurethane foam and steel in a composite to increase the in-plane buckling capacity so that plate stiffeners can be eliminated. The low density foam used in the test composite had little or no tensile strength and insufficient compressive strength to be beneficial structurally. Generally, the tested foam acted as a crack arrest layer but did not function structurally. Therefore, the desired crack arresting structural composite configuration was not achieved. The tested foam possessed some energy absorption capacity; however, this capacity was small when compared to that of the steel in membrane action. The foam lessens the localized straining of the steel plates around a concentrated load point which delays, but does not prevent, the shear tension failure of the steel hull plates.
Structural and performance problems associated with hulls of product oil tankers are also applicable to many other civil and maritime structures. For example, orthotropic steel bridge decks have a reduced fatigue life due to the rupture of the welds that join the deck plate with the stiffening elements. These welds are subject to cyclic concentrated leads from traffic which cause large localized transverse stresses in the weld as the deck plate bends over the stiffening elements.
Thus, a need exists in the art for a hull construction system that simplifies the complexity of hull structure, reduces fabrication and maintenance costs, and increases energy absorption capacity and plastic behavior in the event of accidental or extreme loads to reduce or eliminate cargo loss due to hull rupture and crack propagation.
Thus, a need also exists in the art for steel construction of civil and maritime structures that simplifies construction, and improves structural performance and the life of the structure.
The above-described drawbacks inherent in the art for providing double hull tankers are advantageously eliminated in accordance with the teachings of the present invention by bonding a tough structural elastomer between steel plates to form steel-elastomer-steel composite hull panels, frames and supporting members. The elastomer is preferably hydrophobic to prevent water absorption which could lead to rusting of the plates and should have sufficient ductility to exceed the yield strain of the steel plates without rupturing. The composite panels are used in constructing at least the inner hull of the double hull. Preferably the steel-elastomer-steel composite panels are used to construct the inner hull, outer hull, bulkheads, floors, decks and collapsible frame and support members and may be formed in any necessary shape. The elastomer layer within the composite panels forming the inner hull particularly provides an effective crack arrest layer between the inner steel plate of the inner hull and the outer steel plate of the inner hull, which effectively isolates the inner steel plate of the inner hull from cracks that propagate from the outer hull, the transverse members, such as floor frames and bulkheads, and other supporting elements, such as web frames and horizontal frames, that are designed for both in-service loads and for accidental or extreme loads. Furthermore, because the composite panels are stronger and stiffer than conventional steel plates, the number of framing and supporting elements can be significantly reduced while meeting or exceeding current design standards for strength, service life, construction cost, maintenance cost and survivability. Furthermore, the composite panels may be modified to include voids within the core to lighten the structure or to enhance performance. The shapes, location pattern and orientation of these voids may be optimized to provide the required performance for the particular structure. The voids may be formed with prefabricated rigid foam forms or with forms constructed of light gauge steel or of other materials that are suited to the structure and compatible with the elastomer. Structural shapes or plates may be incorporated to enhance shear stiffness. The connection and shear transfer between these structured shapes and the face plates is in one embodiment provided by bonding the polyurethane elastomer core material to both the structured shape and the face plates without welding. Structural bonding of the face plates or of the face plates and internal structural shapes or plates may be sufficient to provide adequate shear transfer and eliminate the need for welding between metal parts or surfaces. This attribute is particularly advantageous when dissimilar metals are required to be used for the face plates.
In accordance with the teachings of the present invention, a composite steel polyurethane elastomer Sandwich Plate System (xe2x80x9cSPSxe2x80x9d) with or without voids and with properly detailed floor and transverse bulkheads is particularly suited for use in containment vessels such as, for example, oil tankers, or for other civil or maritime structures, such as, for example, orthotropic bridge decks or other traditionally stiffened steel plate structures. These vessels or structures can be fabricated to substantially eliminate the drawbacks associated with known all steel vessels or structures. The specific details relating to ship design may be found in American Bureau of Shipping and Affiliated Companies, 1996 Part 3, Hull Construction and Equipment; Part 5, Specialized Vessels and Services, which is incorporated herein by reference.